With progress in control automation, there is an increasing demand for safety and reliability in an electronic control apparatus. In order to ensure the safety of the electronic control apparatus, it has been required to detect an abnormality immediately after the abnormality is generated and to stop operations. A relay is used in many cases for detaching the apparatus in the event of an abnormality. For example, a main power supply may be cut with a relay in the event of an abnormality. In a motor driving apparatus (particularly, in an electronic steering system), a driving current output to a motor (a phase output) may be cut off by use of a relay in the event of an abnormality.
At the same time, an attempt has been in progress for the sake of higher reliability, longer life, downsizing, and speeding up of protective operations by replacing the relay with a semiconductor device.
Patent document 1 given below discloses a technology for detaching a motor and an inverter from each other. In accordance with the technology, a MOSFET (Metal-Oxide-Semiconductor Field-Advantage Transistor) is inserted into the phase output of a power converter in an electronic steering system. Then, by turning off the MOSFET in the event of an abnormality, the motor and the inverter are detached from each other.
FIG. 4 of patent document 2 given below shows a technology in which two MOSFETs are provided at a location between a power supply and a power converter in such a way that the diode orientations are placed in directions opposite to each other. (To put it concretely, the flowing directions of currents are directions which are outward to each other). With the MOSFETs used as a relay, even if the MOSFETs are put in a power OFF state, parasitic diodes of the MOSFETs cause currents to flow. Thus, in patent document 2, the two MOSFETs are connected to each other in series in such a way that the orientations of the parasitic diodes are put in directions opposite to each other. (To put it concretely, the flowing directions of the currents are directions which are outward or inward to each other).
(FIG. 2 of) patent document 3 given below shows a technology in which a charge-pump power supply is cut off with a relay to reliably power off a MOSFET.